Fig. 1. Permeable and impermeable fluorophores for protein labelling. (A) A self-labelling tagged transmembrane protein is localized at the cell surface in addition to different intracellular, membrane-enclosed compartments (e.g. vesicles, ER) and is visualized by staining with a fluorescent dye. Unbound dyes lead to non-specific background signals. (B) Sulfonation prevents the dye from entering the cell and therefore cleanly labels surface proteins, isolating them for visualization/interrogation.